Etudes et évaluation de processus océaniques par des hiérarchies ...

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9.3. WHAT DRIVES THE THERMOHALINE CIRCULATION? 55
overturning (q)
1 2 3
forcing (P)
tel-00545911, version 1 - 13 Dec 2010
Figure 9.4: Hysteresis behaviour of the model as described by eqs. (9.12) and (9.13). The
strength of the overturning circulation (q) is described as a function of the precipitation (P)
for a fixed temperature difference (∆T). Stable solutions are sown in red, the unstable solution
in blue. There is one solution in the region 1 and 3. There are two stable solutions and one
unstable solution in region 2.
9.3 What Drives the Thermohaline Circulation?
A key question we have not considered so far is where does the mechanical energy come from
that drives the thermohaline circulation and transports the heat? A question we did not consider
when discussing the Stommel-Aarons model, which is based on conservation of potential
vorticity. The evaporation takes water from the surface which is then, at a different location,
reintroduced in the ocean, by rain and river runoff. The important point is, however, that
the mass is taken and put back at the ocean surface, that is at the same geopontential height!
Which means that no net potential energy (mgh) is provided to the ocean as neither mass
nor height is different at evaporation and precipitation points (see fig. 9.3). What about the
mechanical work (dW = −pdV ) done on the ocean by thermal expansion and contraction, that
is change of volume dV . Again, both processes happen at the surface, at the same pressure
p, and again: no net energy is provided to the ocean by thermal atmospheric forcing. Please
note that the situation is completely different for the atmosphere, as shown in fig. 9.3, which
is generally heated at a lower geopotential height, typically at the surface, than at which it is
cooled, by rayonating energy into space, and mechanical work is provided.
So what drives the THC? Originally it was thought that the forcing comes from the cooled
water pushing the the thermohaline circulation until Sandström, in 1908, asked the question
about the energy balance discussed above. Sandström concluded that in a fluid heated and
cooled at the surface the fluid below the cold source, should be homogeneous at the cold
temperature and the fluid between the cold and warm sources would be stably stratified with
only low fluid velocities. A result that bears the name of Sandström’s Theorem.
Then the idea was put forward, that the diffusion of heat from the surface into the depth
at low latitudes descends the effective heating into the ocean and provides thus for the missing
energy to drive the THC, which meant that the THC is pulled rather than pushed. Recent
research initiated by Munk & Wunsch in 1998 favors still another idea, which is that the driving
force is the wind. This means that the low to high latitude heat flux of 2 × 10 15 W is a passive
consequence of the wind driven circulation powered by only 2 × 10 12 W, a thousand times less!